VANCOUVER — Seven hours — not more and not less — appears to be the magic number when it comes to how much sleep we need to keep our brains sharp and to possibly avoid mental decline or even Alzheimer’s disease, according to researchers of new studies presented in Vancouver Monday.

The preliminary (unpublished) research was discussed at the Alzheimer’s Association International Conference, being held here this week for about 4,000 researchers and Alzheimer’s experts.

Too much sleep — either all at once or including naps during the day — may be just as bad as too little sleep; both are associated with mental decline. People who have disrupted sleep because of snoring and sleep apnea, or daytime sleepiness, are also more likely to experience mild cognitive impairment or dementia.

But whether sleep problems are a cause of mental decline, an effect, or both, is still a matter open for more study, debate and proof.

“Is it a chicken or egg scenario? The truth is, we don’t yet know, and at this point, my guess is it’s bi-directional,” said Elizabeth Devore, conceding that sleep problems aren’t exactly uncommon in those over 65. Indeed, more than half over that age have sleep disturbances which are also associated with many other health conditions like depression, stroke and cardiovascular disease — also risk factors for dementia.

However, Devore, of Brigham and Women’s Hospital in Boston, and other researchers, said in all their studies, they took into account (controlled for) those confounding factors, and yet they still came up with results showing that sleep and cognitive decline are linked.

“What we can say is that extreme sleep durations (too little or too much) may contribute to cognitive loss,” she said.

The research conducted by Devore and colleagues, was based on data from more than 120,000 nurses who were aged 30 to 55 when they enrolled in the Nurses’ Health Study. They have answered questionnaires every few years and queries about sleep were added in 1986 and 2000. Cognitive testing was also done between 1995 and 2001 when the nurses were all at least age 70.

The researchers found that nurses who slept five hours a day or less, had lower average memory and cognition scores than those who slept seven hours a day.

Those who slept at least nine hours a day also had lower cognition scores than those who slept seven hours.

An analysis of the results revealed that women whose sleep changed by at least two hours a day had worse mental function than those with no change in sleep time.

Dr. Kristine Yaffe, of the University of California (San Francisco) said in other studies on more than 1,300 women over age 75, those with sleep apnea or some other disordered breathing, had more than twice the odds of developing cognitive impairment or dementia over a five-year study period, compared to those who didn’t have such breathing problems.

Women who had greater nighttime wakefulness, or insomnia, were more likely to get worse scores on cognition and verbal fluency tests. Sleep apnea (suspension of breathing during sleep) and snoring — which a third of the elderly experience — both decrease oxygen levels in the blood, causing a state called hypoxia.

Yaffe said she thinks hypoxia deserves far more study in the context of dementia. Referring to an unrelated study, she noted that in resuscitated cardiac arrest patients, there was a rise in the immediate aftermath period of blood amyloid levels. Amyloid is a protein that accumulates in the brains of Alzheimer’s patients, causing sticky plaques. There may be a connection, then, between oxygen deprivation from something as banal as snoring, and dementia.

In light of the new findings, Devore said doctors should be assessing patients for sleep problems and referring them to sleep specialists who may be able to prescribe treatment to perhaps delay or prevent dementia. Machines called CPAPs have been shown in some small studies to improve cognition, she noted.

Presenting results from an ongoing (eight years) three city study done on nearly 5,000 in France, Dr. Claudine Berr told delegates that excessive daytime sleepiness, often necessitating napping, was an independent risk factor for cognitive decline.

By contrast, difficulty staying asleep, was not associated with cognitive decline; in fact it appeared to have a protective effect, according to data from the National Institutes of Health and Medical Research (INSERM) study.

“It may be that excessive sleepiness, which was shown in our study to be associated with a 30 per cent increase risk of cognitive decline, may be due to early stage brain lesions in areas (of the brain) associated with circadian rhythm abnormalities,” she said.

During the question and answer session with delegates, an American neurologist noted that some patients prescribed melatonin experience vastly improved sleep patterns. So instead of prescribing hypnotics (sleeping pills), doctors should think about recommending melatonin instead because of its safer profile.

There was general agreement that studies on melatonin and dementia prevention should be done and that doctors also need some standard prescribing guidelines for melatonin, a hormone supplement that some use for jet lag and insomnia.